На берегу дети нашли перо чайки, а потом увидели в траве маленького жука.

Because берег often uses a special location form after в and на: на берегу = on the shore / at the shore.

This form in -у is often called the locative or second prepositional. Russian has a small group of masculine nouns that do this:

• в лесу = in the forest
• на полу = on the floor
• на берегу = on the shore

So here на берегу is the normal way to say location.

Yes. Дети is the irregular plural of ребёнок.

Russian has a few common nouns with irregular plural forms, and this is one of the most important ones:

• singular: ребёнок = child
• plural: дети = children

So дети нашли means the children found.

Because they are past tense plural forms.

In the Russian past tense, verbs agree with the subject in number, and in the singular they also agree in gender:

• он нашёл = he found
• она нашла = she found
• они нашли = they found

Here the subject is дети, which is plural, so both verbs are plural:

• дети нашли
• дети увидели

The ending -ли is the normal plural past ending.

Because the sentence describes completed events in sequence:

1. the children found something
2. then they saw something else

Russian typically uses the perfective aspect for this kind of one-time, completed action in a story.

• нашли is from найти = to find, perfective
• увидели is from увидеть = to see, notice, catch sight of, perfective

If you used imperfective forms, the meaning would shift more toward process, repetition, or background action.

It is a noun + noun structure:

• перо = feather
• чайки = of a seagull

So literally it means a feather of a seagull or a seagull’s feather.

More specifically:

• перо is the direct object of нашли
• чайки is genitive singular, showing possession or association

This is very common in Russian:

• крыша дома = the roof of the house
• хвост собаки = the dog’s tail
• перо чайки = a seagull’s feather

Also, перо is neuter and inanimate, so its accusative form is the same as its nominative form.

By itself, чайки can be ambiguous, because it can be:

• genitive singular = of a seagull
• nominative plural = seagulls

But in this sentence, the grammar makes it clear that it is genitive singular, because it depends on перо:

• перо чайки = a seagull’s feather

So here it does not mean seagulls.

Because this is location, not motion into something.

Russian uses different cases after в depending on the meaning:

• в траве = in the grass, inside the grass area, location
• в траву = into the grass, movement toward it

In this sentence, the children saw the beetle in the grass, so Russian uses the prepositional case:

• в траве

Because it is the direct object of увидели, so it must be in the accusative case.

For animate masculine singular nouns, the accusative looks like the genitive:

• nominative: маленький жук = a small beetle
• accusative: маленького жука = a small beetle, as the object

The adjective changes too:

• маленький → маленького
• жук → жука

This is a very important rule in Russian:

• animate masculine accusative = genitive
• inanimate masculine accusative = nominative

Compare:

• Я вижу маленького жука = I see a small beetle
• Я вижу маленький стол = I see a small table

А here links the two parts of the sentence and gives a natural narrative transition: and then / and after that.

It does not have to mean strong contrast. Very often, а simply marks a shift to the next piece of information:

• first they found a feather
• а потом they saw a beetle

So а потом sounds very natural in storytelling. It is a bit more structured than just потом, and less neutral than plain и.

Because Russian has no articles.

So nouns like перо, чайки, and жука do not automatically show a or the the way English does. Russian expresses that idea through:

• context
• word order
• what has already been mentioned
• general meaning of the sentence

So depending on context, перо чайки could be understood as:

• a seagull feather
• the seagull’s feather
• a feather of a seagull

And маленького жука could be a small beetle or the small beetle. In this sentence, English would usually translate them as a feather and a small beetle.

Russian word order is more flexible than English word order because case endings show grammatical relationships.

Starting with На берегу helps set the scene first:

• On the shore, the children found...

That sounds very natural in Russian. The sentence then does something similar later with в траве, placing the location before the object:

• увидели в траве маленького жука = saw a small beetle in the grass

So the word order is not random. It helps guide attention:

1. location
2. action
3. thing found/seen

That is a very common Russian pattern in narration.